The ancient city of Holmul isn’t much to look at. To the casual observer it’s just a series of steep, forested hills in the middle of the jungle in northern Guatemala, near the Mexican border. The jungle here in the Petén Basin is thick and warm but drier than you might expect. And silent, except for the drum of cicadas and the occasional calls of howler monkeys.

Take a closer look, and you may notice that most of these hills are arranged in massive rings, like travelers huddled around a fire on a cold night. An even closer look reveals that parts of the hills are made of cut stone, and some have tunnels carved into their sides. In fact they’re not hills at all but ancient pyramids, left to decay after the collapse of the Maya civilization a millennium ago.

The site was a thriving settlement during the Classic Maya period (A.D. 250-900), a time when writing and culture flourished throughout what is today Central America and southern Mexico. But it also was a time of political upheaval: Two warring city-states were locked in perennial conflict, grappling for supremacy. For a brief period one of those city-states prevailed and became the closest thing to an empire in Maya history. It was ruled by the Snake kings of the Kaanul dynasty, which until just a few decades ago no one even knew existed. Thanks to sites around this city-state, including Holmul, archaeologists are now piecing together the story of the Snake kings.

Holmul isn’t a big, famous site like nearby Tikal, and it was mostly ignored by archaeologists until 2000, when Francisco Estrada-Belli arrived. An Italian-born Guatemalan, he’s ruggedly handsome with scruffy hair and a relaxed demeanor. He wasn’t looking for anything fancy, such as Classic-era written tablets or ornate burials—just some insight into the roots of the Maya. One of the first things he found was a building a few miles from what appeared to be Holmul’s central cluster of pyramids. In it were the remnants of a mural portraying soldiers on a pilgrimage to a faraway place.

Oddly, parts of the mural had been destroyed, apparently by the Maya themselves, as if they’d wanted to erase the history it depicted. Hoping to understand why, Estrada-Belli tunneled into several nearby pyramids. Ancient Mesoamericans built their pyramids in stages, one on top of the other, like Russian nesting dolls. When the people of Holmul added a new layer, they preserved the one beneath, which has allowed researchers to tunnel in and see previous structures almost exactly as they were left.

In 2013 Estrada-Belli and his team worked their way into one of the larger pyramids, tracing an ancient staircase to the entrance of a ceremonial building. Climbing up through a hole in the floor, they discovered a 26-foot-long frieze, marvelously preserved, above the entrance to an ancient tomb.